I finally got the Grohe 1/2" Compression Cartridges (Part # 07-147-000) removed which was installed in 1984 - after 30 years. My original plumber never informed me that I needed to replace the compression cartridges. I only found out when I left it unused for a month and then barely a trickle of water came out of the shower head.
My current plumber could not get it removed and told me to replace the entire Grohe shower handles unit with another brand.
But I love my Grohe shower handle fixtures (the water pressure is so good) so why spend the additional money to replace it.
Since the plumber could not turn the compression metal cartridges -- This is what I did to get the compression cartridges to turn!!
First I put CLR calicum remover (or use any good brand) with a rag and soaked the rag with the CLR solution on it. Then I wraped the rag around the old compression cartridges and then putting a heavy plastic bag over the rag to keep the solution onto the old compression cartridges. Then I put a rubber band wrapped around the lock bag to keep the CLR soluation onto the old compression metal cartridges. I then kept this on for 1 month with the CLR solution on the old compression cartridge; Making sure that the CLR solution was not dripping onto the shower tiled floor by putting a towel under it. It will drip onto the shower walls so you have to watch it so that the CLR solution does not stain your walls.
Then I used SiliKroil (which is only bought on-line) which an Airline Mechanic told me when I was a flight attendent -- which is a Kroil's high solvency penetrating oils first penetrate to one-millionth inch spaces to break the bond of rust (but mine was calcium build-up) and to provide lubrication at the first molecular level. This frees the frozen metal part. The dimethyl silicones then provide extra long-lasting lubrication and rust protection. You can also buy the original Kroil which does not have silicone so that it does not affect your seals.
I sprayed SiliKroil it on and waited two days, while constantly spraying the SiliKroil penetrating oil onto the old compression metal cartridges. Make sure that it does not stain your shower floor or walls by wiping it off immediately.
Then on the third day, the old compression cartridges finally turned and I could remove the old cartridges off!! It took time to turn the cartridges and you just have to be patient and carefully not to rush turning the cartridges so that they don't break off. Then it was easy to hand turn the new compression cartridges into the handles -- remember your particular compression cartridge there may be a left cartridge turn and a right cartridge turn!